Kassnar-Churchill O/U problem

I have a customer that has a Kassnar-Churchill single trigger O/U 12 ga. that missfires on occasion. The trigger "clicks", but it doesn't fire. I have not looked at it yet, but he has taken it to other competent gunsmiths, and it came back, and worked for about 50 shots, then started randomly misfiring again. I assume they cleaned it good, and thought that solved the problem.

I checked around for parts, and found a listing at Numrich, but some parts are out of stock, and there are no exploded views. I have one vague schematic.

The first place i'd look is the hammer springs (if it has hammers, if not striker springs) It's very possible they've worn out. The fact that there's nothing available for these guns it's possible the other gunsmiths found this and simply stretched the springs to get the gun by. It may be an issue of having to make replacements. On the other hand it might be time to sell your customer a new shotgun.